The compound D-isoglutamyl-D-tryptophan (also known as H-D-iGlu-D-Trp-OH or Thymodepressin) is a synthetic hemoregulatory dipeptide having the following formula:

Thymodepressin is the free diacid having Chemical Abstracts Service (CAS) Registry Number® of 186087-26-3. U.S. Pat. No. 5,736,519 discloses H-D-iGlu-D-Trp-OH and a process for its preparation wherein it is purified by ion exchange chromatography. It is an immunosuppressant and selectively inhibits proliferation of hemopoietic precursor cells and stimulates granulocyte and lymphocyte apoptosis (Sapuntsova, S. G., et al. (May 2002), Bulletin of Experimental Biology and Medicine, 133(5), 488-490).
Thymodepressin is currently being sold in Russia as the disodium salt of D-isoglutamyl-D-tryptophan in liquid formulation for injection and intranasal administration for the treatment of psoriasis and atopic dermatitis. The solid form of the disodium salt of D-isoglutamyl-D-tryptophan is an amorphous powder which is hygroscopic and very difficult to handle. The disodium salt of D-isoglutamyl-D-tryptophan has the molecular formula C16H17N3Na2O5 and the following chemical structure:
which is reported in Kashirin, D. M., et al. (2000), Pharmaceutical Chemistry Journal, 34(11), 619-622.
Through investigations in our laboratory, we have determined that the freeze-dried disodium salt of D-isoglutamyl-D-tryptophan is extremely hygroscopic turning into a gel in a matter of minutes in air and cannot easily be handled.
A powdery or amorphous form of a compound, intended for pharmaceutical use may give rise to manufacturing problems due to bulk density issues, hygroscopicity and variable water content that cannot be corrected by vacuum drying. D-isoglutamyl-D-tryptophan is a dipeptide and the drying of an amorphous form at elevated temperature, for example, 80-100° C. under vacuum is not recommended. Thus, there are serious difficulties experienced during the purification of the disodium salt of D-isoglutamyl-D-tryptophan and obtaining the pure disodium salt on a manufacturing scale. Further, there is no published procedure for its preparation.
The monosodium salt of D-isoglutamyl-D-tryptophan is identified by the CAS Registry Registry System and is listed in the CAS REGISTRYSM File with a CAS Registry Number® of 863988-88-9. However, there are no references citing the substance and thus no publication of its identity, its physical and/or chemical properties, its characterization or a procedure for its preparation. Freeze-dried powders of mono sodium and disodium salts of peptide drugs may not have controllable powder bulk density ranges for formulation. They may require significant investment in freeze-dried dispersion technology.
Therefore, there is a need to develop alternative pharmaceutically acceptable salts of D-isoglutamyl-D-tryptophan which are crystalline. Such crystalline salts can generally be purified more easily than an amorphous form and may possess other advantageous properties, for example in relation to their particular crystalline form and/or their solubility characteristics and/or their lack of hygroscopicity and/or their stability characteristics, including their thermal stability properties and/or their ability to undergo oxidative degradation.